The Orientation of Animals by Opposed Beams of Light

When orientation is attained under the influence of beams of parallel light opposed at 180 degrees the deflection theta from a path at right angles to the beams is given by tan See PDF for Equation, where I(1) and I(2) are the photic intensities and H is the average angle between the photoreceptive surfaces. This expression is independent of the units in which I is measured, and holds whether the primary photosensory effect is proportional to I or to log I. When photokinetic side-to-side motions of the head occur, H decreases with increasing total acting light intensity, but increases if higher total light intensity restricts the amplitude of random movements; in each case, H is very nearly proportional to log I(1)I(2). For beams of light at 90 degrees , See PDF for Equation. The application of these equations to some particular instances is discussed, and it is shown why certain simpler empirical formulae previously found by others yield fair concordance with the experimental data. The result is thus in complete accord with the tropism theory, since the equations are based simply on the assumption that when orientation is attained photic excitation is the same on the two sides.